1. Field of the Invention
This invention relates to a method for the preparation of coffee by means of a coffee brewing apparatus, particularly by means of a program controlled coffee brewing apparatus which comprises a coffee bean milling assembly, a brewing chamber adapted to receive the coffee powder delivered by the coffee bean milling assembly and having an inlet for the coffee powder as well as a beverage outlet, a piston displaceably mounted above the brewing chamber, driving means to displace the piston from a position remote from the brewing chamber whereby its inlet is open to a position in the interior of the brewing chamber whereby its inlet is closed, and a hot water supply assembly connected to the brewing chamber and adapted to deliver hot water under pressure into the brewing chamber.
With such an apparatus, a coffee beverage is prepared as follows: Coffee beans are milled to coffee powder with a preset rate of milling, and a preset amount of coffee powder is filled into the brewing chamber when the piston is in its retracted position. By displacing the piston towards the brewing chamber, the inlet opening thereof is closed and the piston reduces the internal volume of the brewing chamber while being displaced into its interior, thereby exerting a preset pressure on the coffee powder contained in the brewing chamber. Finally, a preset amount of hot water is fed into the brewing chamber and forced to pass through the compressed coffee powder whereby the resulting coffee beverage is collected.
2. Prior Art
The above described method is well known in the art and a plurality of coffee brewing apparatuses exists which are operated according to this method. Examples thereof are disclosed in the Patent Publication No. PCT-WO 82/01120 and in the German Laid Open Publication No. 20 53 466. The general objects to be met thereby are to prepare an aromatic coffee beverage, to efficiently exploit the coffee powder, to avoid the need to perform a plurality of manual operation steps, and to prepare the coffee beverage in different quantities (one cup or two cups simultaneously).
Methods of coffee preparation known in the art usually base on the generally correct assumption that, for the preparation of e.g. one cup of coffee, a preselected amount of coffee powder has to be filled into the brewing chamber, a preselected pressure has to be exerted on the coffee powder in the brewing chamber, and a preselected amount of hot water has to pass the compressed coffee powder. Known coffee brewing apparatuses operating more or less automatically perform this method usually as follows:
1. A coffee bean milling assembly is operated during a certain time period to produce a certain amount of coffee powder, the exact amount being dependent of the time the mill is operated.
2. The (theoretically known and constant) amount of coffee powder thus produced is filled into the brewing chamber which has a known internal volume.
3. A piston is moved into the brewing chamber in order to close it, whereby the displacement path is calculated from the known internal volume of the brewing chamber and the (theoretically known) amount of coffee powder contained therein, in an attempt to compact the coffee powder by exerting a preselected pressure.
4. A preselected amount of brewing water is forced through the compacted coffee powder cake contained in the brewing chamber and the beverage thus produced is collected.
In practice, however, some serious and disadvantageous problems occur:
A constant milling time period of the coffee beans must not unconditionally result in a constant amount of coffee powder. The relation between the milling time period and the resulting amount of coffee powder depends, amongst else, from the kind and quality of the coffee beans used, from the operational condition of the milling assembly (wear etc.) and from fluctuations of the mains supply voltage.
A constant displacement path of the piston must not unconditionally result in a constant pressure on the coffee powder in the brewing chamber with a preselected pressure force. This is particularly true if the amount of coffee powder delivered by the milling assembly varies due to the above mentioned reasons. Furthermore, the preselected displacement path is only theoretically "constant"; in fact, significant deviations from the preselected value may occur due to irregularities in the driving means, wear thereof etc.
The milling rate of the coffee powder, i.e. the fineness or grain size thereof is not taken into account in the methods known in the art. It is obvious that the grain size may unintentionally vary as well, e.g. depending of the kind and quality of the coffee beans, wear of the milling assembly etc. However, a fluctuation in grain size is immediately reflected in an impairment of the quality of the coffee beverage.
The methods of preparing coffee which are well known in the art thus require, even if they are said to be automatically operating, significant expenditures as far as supervising, adjusting of the components of the apparatus, servicing and maintenance are concerned if no fluctuations or impairment of the coffee beverage quality is to be accepted.